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Aerosols in the Atmosphere

By Maricela Reyes

This paper was written as a reference to provide
scientific background to aid the Aerosols team in their research.

Introduction

When solar radiation passes through the atmosphere, it is absorbed and
scattered, not only by atmospheric gases, but also by aerosols and clouds.
The Aerosols research project is concerned with using measurements
of solar radiation to infer properties of aerosols.

What is an Aerosol?

Aerosols are defined as suspensions of liquid or solid particles in the
air, excluding cloud droplets and precipitation. The mean radii of
aerosol particles range from about 10-4 to 10
microns (µm).
Aerosol particles in the atmosphere are mainly due to two processes:

Direct injection into the atmosphere such as the formation of dust,
soot and sea salt particles from human and/or natural process; and

Chemical reactions of gaseous materials within the atmosphere, such
as the transformation of SO2 into
HSO4 or sulfates,
NOx into nitric acids, etc.

Aerosols have different sizes. They can be classified according to their
origin, size, and atmospheric distribution. The very small particles with
mean radius between 0.001 and 0.1 microns are called Aitken particles.
Particles with radii between 1 and 10 microns are considered large or coarse
particles. Particles with radii between 0.1 and 10 microns are
responsible for the turbidity (haziness) of the atmosphere. The
concentration of aerosols is usually greater over the continents than
over the oceans. The concentration of Aitken nuclei ranges from values
of 150000 cm-3 in cities to 400 cm-3
over the oceans. The concentrations of
large and giant particles are much smaller than those of the Aitken particles,
and strongly depend on the type of air mass, being greatest in moist,
tropical air masses.

For most atmospheric applications we reference the size of the aerosol
using the particle radius where in environmental and health research, the
particle diameter is used as a reference. Thus PM 2.5 refers to aerosols
(particulate matter) with diameters less than 2.5 microns.

Why Do We Care About Aerosols?

Aerosols affect planetary energy balance in two ways:

Directly: aerosols scatter and absorb solar energy both in
cloud-free and cloudy conditions; and

Indirectly: via their role as cloud condensation nuclei (CCN),
aerosols modify the optical properties and lifetimes of clouds playing
an important role in the process of cloud formation and precipitation.

What Happens When Light is Absorbed?

Absorption of radiation causes local heating of the earth's atmosphere.
As this happens, the stratosphere is locally heated by the absorption of
aerosols that can generate winds and temperature inversions.

What Happens When Light is Scattered?

Scattering of radiation causes a cooling effect, altering the weather
and climate. Scattering occurs because the index of refraction of the
particles differs from that of the homogeneous medium in which they are
imbedded (Houghton, 1985). Although the frequency of the scattered radiation
does not change, its phase and polarization may change substantially from
those of the incident radiation.

The radiation that finally reaches the surface is partly reflected and
partly absorbed by ocean waters, soil, vegetation, snow, and ice. A large
portion of the latter energy is used to evaporate water into the atmosphere,
whereas the remainder is transferred down into the ocean by conduction and
turbulent heat exchange and up into the atmosphere by similar processes and
by the emission of long-wave radiation. The fraction of the total incident
solar radiation that is reflected and backscattered is called the albedo, as
we have seen before. The albedo of the earth, as measured at the top of the
atmosphere, increases with latitude, and changes seasonally.

What Does Scattering Mean and Why is it Important in the Earth's Atmosphere?

Scattering is a rapid process whereby light is actually absorbed by a
particle and then quickly emitted in another direction. Scattering particles
can be air molecules, water droplets, or pollutants.
The light in the atmosphere is diverted (scattered) from its direction
of propagation when it encounters particles or inhomogeneities, such as air
molecules, aerosols, and clouds (see figure 1 below). It is important
because the materials that scatter light (e.g. clouds) can affect
weather and hence can give us an indication of what is happening in the
Earth's atmosphere.

Figure 1:
How particles in the air scatter sunlight.

The Scattering Process

Scattering of solar radiation does not lead to a conversion of radiant
energy into heat as does absorption. The radiant energy is merely dispersed
in all
directions as if the particles act as a new source of radiation. Because
some of the solar energy is scattered backwards and sideways, the amount of
energy that reaches the surface is partly reduced. Thus, both absorption and
scattering lead to a depletion of solar radiation.

There are 2 kinds of scattering: Rayleigh Scattering and Mie Scattering.

Rayleigh Scattering

Rayleigh developed the scattering theory for light scattered by particles or
molecules in the atmosphere with diameters smaller than the wavelength
of incident light. He showed that the amount of scattering is inversely
proportional to the fourth power of the wavelength (λ-4).
Therefore, the following rule comes into play:

The shorter the wavelength of the incident light, the
more the light is scattered.

Light in the blue part of the spectrum is more intensely scattered than
in the red part by atmospheric molecules, hence we see a blue sky. On
the other hand, sunsets and sunrises appear reddish because the shorter
(blue) wavelengths in direct light are removed by scattering through the
long path in the atmosphere, leaving the remaining reddish colors of the
spectrum (see figure 2 below).

Figure 2:
Sunlight is scattered by particles in the earth's atmosphere.

The illustration above shows that the light arriving at position A
travels a shorter distance through the atmosphere than light arriving
at position B. Therefore, a person at position B would see red light that
encounters more scattering molecules and aerosols due to a longer path.
A person at position A would see blue light because the light travels a
shorter distance through the atmosphere before arriving at the surface of
the earth.

The shorter radiation is also scattered by particles (dust, smoke, and
ions) and impurities that form aerosols. When the dimensions of these
particles increase, the λ-4 law ceases to be valid,
dispersion is less selective with respect to λ,
and Mie scattering
theory (see below for explanation) should be used (Van de Hulst, 1957). Mie
theory is more generally valid and contains Rayleigh scattering and
geometrical optics as limiting cases. When the particles are sufficiently
large, the dispersion of radiation approaches a
1/λ dependence, leading
to diffuse reflection. This explains why cloud drops and ice crystals
reflect or refract radiation in all directions.

In general, for large
particles, the change in direction of the incident radiation may be
explained by geometrical optics, such as diffraction, reflection,
refraction, or a combination of these effects, producing coronae, haloes,
rainbows, etc. The diffuse radiation of "white" sunlight is also white due
to the multiple reflections and refractions, explaining the whitish color of
the clouds. Volcanic eruptions cause colorful sunrises and sunsets due
to the large amounts of aerosols they eject into the air.

Mie Scattering

Mie scattering occurs when the size of the particle is on the order
of the wavelength. Mie scattering works only for spherical particles such
as cloud droplets. This type of scattering is responsible for the white
appearance of clouds because the cloud droplets scatter all wavelengths of
visible light in all directions.

Aerosols and Health

Aerosols can also affect health. Recent epidemiologic studies have
indicated associations between ambient particulate matter and increased
mortality and morbidity. Aerosols may also be linked to the increasing
incidence of asthma.